We think more clearly, feel better overall, and protect ourselves against some of the cognitive decline that occurs with age, studiessuggest.

"Aerobic exercise ... has a unique capacity to exhilarate and relax, to provide stimulation and calm, to counter depression and dissipate stress," the authors of anarticle in the Harvard Medical School blog"Mind and Mood" wrote.

But how long should you be cycling, swimming, walking, or running to reap these benefits?

Overall, research suggests that the magic happens somewhere in the window of about 30-45 minutes at minimum.

Aarecent paperlooked at the exercise habits of hundreds of breast cancer survivors who were experiencing symptoms like "chemo brain," which involves memory loss and trouble focusing. The researchers found that as little as 30 minutes of an aerobic exercise like walking was linked with significantly better performance on cognitive quizzes.

Another study published in May provided some additional support for that research — it found that in adults aged 60-88,walking for 30 minutes four days a weekfor 12 weeks appeared to strengthen connectivity in a region of the brain where weakened connections have been linked to memory loss.

Similarly, apilot studyin people with severe depression found that just 30 minutes of treadmill walking for 10 consecutive days appeared to be "sufficient to produce a clinically relevant andstatistically significantreduction in depression."

Other research suggests it might be better to do cardio for longer. A study in the British Medical Journal found that in adults over 50, the best results for the brain appeared to come from a routine thatcombined aerobic exercises with resistance training(i.e. muscle-building exercises like planks and push-ups) and lasted at least 45 minutes.

Researchers still aren't sure why this type of exercise appears to provide a boost to the brain, but some studies suggest it has to do withincreased blood flow, which provides our minds with fresh energy and oxygen. One recent study in older women who displayed potential symptoms of dementia also found that aerobic exercise was linked with anincrease in the size of the hippocampus, a brain area involved in learning and memory. Another reason might have to do with cardio's ability to helpreduce levels of the body's natural stress hormones, such as adrenaline and cortisol, according to a recent study in the Journal of Physical Therapy Science.

Joe Northey, the lead author of the British study and an exercise scientist at the University of Canberra, said his research suggests that anyone in good health over age 50 should do 45 minutes to an hour of aerobic exercise "on as many days of the week as feasible."